Of course its fraud - against bank number 2. They will end up eating the interchange rate (e.g., if the store pays 3% for the card and you charge $1,000 on card 1 but return it to card 2, the supplier is whole but bank 2 will only get back $970 and have to pay you $1,000). Sounds like theft to me. Let alone the cost of the item that you have to return if the seller has to resell it as used.